1. Field of the Invention
This invention relates generally to compact, light weight printheads and, more particularly, to integral Organic Light Emitting Diode (OLED) printheads.
2. Background
Light emitting diodes (LED) have been used for exposing photosensitive materials such as photographic film or paper of photocopying receptors. The light emitting diodes are usually arranged in a linear array or a number of linear arrays and means are provided for a relative displacement of the photosensitive materials in relation to the array. In this manner, the material is scanned past the array and an area is exposed thereby creating an image.
The light emitted from LEDs diverges quickly and thus reduces the exposing intensity and increases the exposing area. This can lead to a reduction in sharpness of the exposed image and to the possibility of undesired exposure of adjacent areas. The first of these problems is known as reduced pixel sharpness and the second is known as crosstalk. To avoid these difficulties. optical systems are utilized to transmit the light from the LEDs to the photosensitive material without significant divergence. While this approach results in an acceptable printing system, such systems have their size defined by the optical systems and therefore are not as compact as would be desired for a portable print system.
The light emitting diodes used in existing printers (see for example, Shimizu et al., LED Arrays, Print Head, and Electrophotographic Printer, U.S. Pat. No. 6,064,418, May 16, 2000) emit radiation from the surface of a p-n junction (constitute edge emitters) and are typically mounted on a printed circuit board. These characteristics of the LEDs used in previous printers impose constraints on manufacturability and preclude their use in compact contact or quasi-contact printing. Other light source systems (LCDs for example) presently used in printers suffer from similar constraints that preclude their use in compact contact or quasi-contact printing. Innovative designs are needed to satisfy the need for compact printers.
Organic Light Emitting Diodes (OLED), which have been recently developed, (See, for example, the article by S. Forrest, P. Burrows, M. Thompson, xe2x80x9cThe Dawn of Organic Electronicsxe2x80x9d, IEEE Spectrum, Vol. 37, No, 8, pp. 29-34. August 2000) hold a promise of ease of fabrication and low cost and low power consumption. A recent publication (Y. Tsuruoka et al., xe2x80x9cApplication of Organic Electroluminescent Device to Color Print Headxe2x80x9d, SID 2000 Digest, pp. 978-981) describes a print head utilizing OLEDs. The printhead described in this publication comprises discrete OLEDs, color filters and optical elements and therefore is not as compact as desired. Also, the presence of discrete optical elements requires considerations of alignment which have an impact on manufacturability and cost.
It is the primary object of this invention to provide an integral printhead which is compact and light weight and utilizes Organic Light Emitting Diodes (OLED). It is a further object of this invention to provide an integral printhead which avoids crosstalk while providing the necessary pixel sharpness and utilizes Organic Light Emitting Diodes (OLED). Other objects of this invention will become apparent hereinafter.
The present invention achieves the stated objective by means of a printhead comprising an Organic Light Emitting Diodes (OLED) structure, where the OLEDs emit radiation over at least three separate wavelength ranges, and where the printhead is designed for contact or quasi-contact printing with the desired pixel sharpness and reduced crosstalk. The OLED structure comprises either actively addressable or passively addressable OLED elements.
In one embodiment, the printhead comprises a transparent substrate having a planar light receiving surface opposite to a planar light emitting surface, and an OLED structure comprising at least one array of OLED elements deposited onto the light receiving surface of the substrate. Two possible different arrangements for the printhead are disclosed. In one arrangement. each OLED array in the printhead comprises at least one of a plurality of triplets of OLED elements, and each element in each triplet being capable of emitting radiation in a distinct wavelength range different from the distinct wavelength range of the other OLED elements in the same triplet. In the second arrangement, the printhead comprises at least one of a plurality of triplets of elongated arrays of individually addressable Organic Light Emitting Diode (OLED) elements, each array in the triplet being aligned in substantially parallel relation to any other array in the triplet, each OLED array in each triplet having elements that are capable of emitting radiation in a distinct wavelength range different from the distinct wavelength range of the other two arrays in the triplet.
In another embodiment, the printhead comprises a substrate having a planar first surface opposite to a planar second surface, an individually addressable Organic Light Emitting Diode (OLED) structure, comprising at least one elongated array of individually addressable Organic Light Emitting Diode (OLED) elements and deposited onto the first surface of the substrate, and a substantively transparent layer, deposited onto the OLED structure, having a light receiving surface in effective light transmission relation to the OLED structure, the light receiving surface located opposite to a light emitting surface. Again, the same two alternative printhead arrangements are disclosed for this embodiment.
The parameters including the distance between OLED elements, the characteristic dimensions of the OLED elements, the distance between the OLED elements and the photosensitive material, are selected to optimize the exposure of the photosensitive material at a given pixel area, corresponding to a given OLED element, due to the light intensity from the elements of the array which are adjacent to the given element and from the given OLED element. An exposure is optimized if the Subjective Quality Factor (SQF) of the resulting pixel is as close to 100 as possible and if the intersection of the normalized intensity profile produced by an adjacent OLED element at given pixel locations with the normalized intensity profile produced by the corresponding OLED element is as close to 0.5 as possible.
In order to reduce crosstalk, a border surrounding an unimaged, colorless area equal to the OLED element area and aligned with the OLED element can be created for each of the OLED elements. Such a border would substantially absorb radiation in all three distinct wavelength ranges and could be formed using an imageable material or a material such as an ink or a dye.
The printheads of this invention can be used to expose the entire gamut of photosensitive materials, for example. silver halide film, photosensitive paper, dry silver, photocopyng receptor material, imageable materials comprised of dyes and acid amplifiers and other photosensitive compounds.
These embodiments provide printheads that are light weight and compact, where an OLED structure is deposited onto substrates and, the printheads are designed for direct quasi-contact printing, without additional optical elements, with the desired pixel sharpness and reduced crosstalk. By virtue of their compactness and their light weight, as well as the low power requirements of OLED elements, the printheads of this invention enable the construction of portable printing devices for the mobile data environment.